TECHNICAL FIELD The present invention relates to a wave selection type diffractive optical element and an optical pickup device having the same.

BACKGROUND ART An optical pickup device is used to record or read information in or from an information recording surface of an optical recording medium such as an optical disc including a compact disc (CD) and a digital versatile disc (DVD), or a magnetic disc. In the optical pickup device, it is required that a focused laser light beam do not deviate from one of tracks formed on the information recording surface of the optical disc in order to rotate the optical disc with the laser light focused on one of the tracks. To implement this, various tracking methods have been developed. One widely used method is a three-beam method of splitting laser light into a main beam that is 0-th order diffracted light and two sub-beams that are ± 1st order diffracted light. At this time, a diffractive optical element is used to split the laser light into the main beam and the sub-beams. This diffractive optical element is disposed on a light path of the laser light. Diffraction occurs at a diffraction lattice region that is a characteristic structure of the diffractive optical element, thereby splitting the laser light into one main beam and two sub-beams. Recently, with the development of technologies, a CD and DVD compatible optical pickup device is being widely used to record and reproduce information in and from both optical discs of CD and DVD. As for the CD type optical disc such as a typical CD-R5 laser light of a 790 nm wavelength band (hereinafter, referred to as "CD type wavelength (λ2)") is used. As for a DVD type optical disc, laser light of a 650 nm wavelength band (hereinafter, referred to as "DVD type wavelength (X1)") is used. Such a CD and DVD compatible optical pickup device needs a X1 laser generator for generating laser light of the DVD type wavelength (λθ and a λ2 laser generator for generating laser light of the CD type wavelength (λ2) as laser light sources, and three- beam generating diffractive optical elements that correspond to the respective laser generators, thereby increasing the size of the optical pickup device. Recently, in order to miniaturize the optical pickup device, a two-wavelength semiconductor laser is being used as a laser light source that generates laser light of the DVD type wavelength (X1) and the CD type wavelength (λ2). Further, a wave selection type diffractive optical element is being used as the three-beam generating diffractive optical element corresponding to the two-wavelength semiconductor laser, wherein the diffractive optical element selectively generates any one of laser light of the DVD type wavelength (λj) and laser light of the CD type wavelength (λ2) into three beams (i.e., the main beam and the sub-beams). The wave selection type diffractive optical element selectively diffracts light of any one of the two kinds of laser wavelengths. A core technique thereof is that laser light undergoes diffraction only in a specific polarization direction when it passes through diffraction lattices formed in the diffractive optical element. An optical device 800 having such a wave selection type diffractive optical element function is disclosed in Korean Patent Laid-Open Publication No. 2001-0089321, as shown in Fig. 1. In the structure of the optical device 800, thin organic films are fixed with an adhesive between fixed substrates 804 and 805. Polarizing diffraction lattices 807a and 807b, which are made of birefringence polymer liquid crystals, are fixed with a filling adhesive 806 between the fixed substrate 805 and a fixed substrate 809. Here, the thin organic films transmit a linearly polarized light of a X1 =650 nm wavelength band, which is one of among incident two-wavelength laser light, with a polarization plane thereof rotated by 90° but transmit a linearly polarized light of a λ2=790nm wavelength band without rotation of a polarization plane thereof. As a result, the polarization planes of the two-wavelength (λi and λ2) laser light passing through the thin organic films become orthogonal to each other. Meanwhile, an orientation direction of polymer liquid crystal molecules in the diffraction lattices 807a is set such that the diffraction lattices 807a have an ordinary refractive index to allow a diffracted light to be generated from light of λ2 passing through the thin organic films rather than light Of X1 orthogonal to λ2. An orientation direction of the polymer liquid crystal molecules in diffraction lattices 807b is set to be orthogonal to that of the diffraction lattices 807a, such that the diffraction lattices 807b have an extraordinary refractive index to allow a diffracted light to be generated from the light of λj passing through the thin organic films rather than the light of λ2 orthogonal to X1. Further, the filling adhesive material, which is filled between the fixed substrates 805 and 809 with the diffraction lattices 807a and 807b formed thereon, has substantially the same refractive index as the ordinary refractive index of the polymer liquid crystals. As shown in Fig. 2, the optical device constructed as above is disposed on a light path between a light source 901 for generating laser light and an objective lens 903 for focusing the laser light onto an optical disc in the CD and DVD compatible optical pickup device 900. The optical device independently generates a main beam that is 0-th order diffracted light and two sub-beams that are ±lst order diffracted light from the two- wavelength (λj and λ2) laser light so that the optical pickup device 900 records or reproduces information on or from any one optical recording medium A of the CD and the DVD. However, such a wave selection type diffractive optical element for the conventional optical device is difficult to manufacture because the diffractive optical element uses a high-advanced technique to orient the polymer liquid crystal molecules in a certain direction. Further, production costs of the optical device and the optical pickup device using the same increase because the optical device needs an expensive optical alignment apparatus to orient polymer liquid crystal molecules during a manufacturing process and the manufacturing process is complicated. Further, because the refractive index of the filling adhesive material is merely set to be the nearly same as the ordinary refractive index of the diffraction lattices, there are problems in that, due to deviation in the refractive index, limitations on improvement of diffraction efficiency and deviation in the quality of products are produced, thereby degrading the reliability of the products. DISCLOSURE OF INVENTION

TECHNICAL PROBLEM Accordingly, an object of the present invention is to provide a wave selection type diffractive optical element and an optical pickup device having the same, which can be easily manufactured and in which production costs thereof can be reduced and deviation in a refractive index can be minimized to improve the quality of the products.

TECHNICAL SOLUTION The object of the present invention is achieved by a wave selection type diffractive optical element for selectively diffracting light of at least one of X1 and λ2 wavelengths belonging to different wavelength bands according to an aspect of the present invention. The optical element comprises at least one light-transmitting transparent substrate; corrugated diffraction lattices that are formed on a surface of the transparent substrate and formed of an anisotropic and non-liquid crystal-based polymer material with birefringence in which molecules thereof are not oriented; and an adhesive material that is filled in at least grooves of the corrugated diffraction lattices and has a refractive index equivalent to or different from that of the diffraction lattices with respect to any one of the X1 and λ2 wavelengths. The transparent substrate may comprise a pair of transparent substrates, the diffraction lattices may be formed on a surface of any one of the transparent substrates, and the adhesive material may be filled between the both transparent substrates. Alternatively, the transparent substrate may comprise a pair of transparent substrates; the diffraction lattices may be formed on a surface of a transparent substrate of the both transparent substrates, which is placed on a X1 and X2 wavelength exit side; a phase plate may be disposed between a transparent substrate of both transparent substrates, which is positioned on a λj and λ2 wavelength incidence side, and the diffraction lattices to retard a phase so that one of the X1 and λ2 wavelengths intersects the other; and the adhesive material may be filled between the transparent substrates. Alternatively, the transparent substrate may comprise a first transparent substrate, a second transparent substrate, and a third transparent substrate, which are sequentially arranged from a Xi and X2 wavelength exit side to a X1 and X2 wavelength incidence side; the diffraction lattices may be formed as first and second diffraction lattices on surfaces of the first and second transparent substrates, the surfaces being directed to the X1 and λ2 incidence side; a phase plate may be disposed between the second transparent substrate and the third transparent substrate to retard a phase so that one of the X1 and λ2 wavelengths intersects the other; and the adhesive material may have a first adhesive material filled between the first transparent substrate and the second transparent substrate, a second adhesive material filled between the second transparent substrate and the phase plate, and a third adhesive material filled between the phase plate and the third transparent substrate. At this time, the diffraction lattices may be formed of ultraviolet curable and thermal curable polyimide. The diffraction lattices may be disposed with a uniform lattice width or non- uniform lattice widths equal to or less than 30 μm and with an equal pitch or unequal pitches between the lattices equal to or less than 60 μm. The adhesive material may be at least one of acryl, epoxy, urethane, and polyester-based resins. Preferably, the phase plate has an optical pass difference value of 1600 nm ±20 nm. The object of the present invention is achieved by an optical pickup device including a light source for emitting light of X1 and λ2 wavelengths belonging to different wavelength bands, an objective lens for focusing the light of the X1 and X2 wavelengths onto an optical recording medium, and an optical detector for detecting the light of the X1 and X2 wavelengths reflected by the optical recording medium according to another aspect of the present invention. The optical pickup device comprises the wave selection type diffractive optical element according to any one of claims 1 to 8 disposed on a light path between the light source and the objective lens.

ADVANTAGEOUS EFFECTS According to the present invention, there is provided a wave selection type diffractive optical element and an optical pickup device having the same, which can be easily manufactured and in which production costs thereof can be reduced and deviation in a refractive index can be minimized to improve the quality of the products.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of a conventional optical device having the function of a wave selection type diffractive optical element. Fig. 2 is a schematic view showing the structure of a typical CD and DVD compatible optical pickup device. Figs. 3 and 4 are sectional views of a wave selection type diffractive optical element according to a first embodiment of the present invention. Figs. 5 and 6 are sectional views of a wave selection type diffractive optical element according to a second embodiment of the present invention. Fig. 7 is a sectional view of a wave selection type diffractive optical element according to a third embodiment of the present invention.

<Explanation of reference numerals for designating main components in the drawings> 1, 1', 1": Diffractive optical element 10, 10', 11, 11': Transparent substrate 10": First transparent substrate 12": Second transparent substrate 13": Third transparent substrate 20, 20' : Diffraction lattices 20": First diffraction lattices 22' : Second diffraction lattices 30, 30': Adhesive material 30": First adhesive material 32": Second adhesive material 33": Third adhesive material 40', 40": Phase plate

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. Figs. 3 and 4 are sectional views of a wave selection type diffractive optical element according to a first embodiment of the present invention. As shown in Figs. 3 and 4, a wave selection type diffractive optical element 1 according to the first embodiment of the present invention has a structure in which diffraction lattices 20 are formed on one transparent substrate (e.g., 10) of a pair of light-transmitting transparent substrates 10 and 11, and an adhesive material 30 is filled between the transparent substrates 10 and 11. A surface of the transparent substrate 10 is coated with an anisotropic and non- liquid crystal-based polymer with birefringence to form a lattice forming layer thereon by means of spin coating or the like and is then patterned to have the corrugated diffraction lattices 20 at predetermined intervals thereon by means of photolithography or ultraviolet (UV) embossing, Hot embossing, or the like. At this time, molecules in the lattice-forming layer of the polymer, which has been just coated on the surface of the transparent substrate 10 (i.e., before the diffraction lattices 20 are formed), have an anisotropic property by which refractive indexes of the molecules are irregular according to polarization directions of λi and λ2 wavelength light. The molecules in the lattice forming layer with the diffraction lattices 20 formed therein are in a medium state where they are randomly arranged and have respective effective refractive indexes (N0' and Ne') according to the polarization directions of the λi and λ2 wavelength light (ordinary or extraordinary light) even when the molecules have not been oriented. Accordingly, if the refractive index of the adhesive material 30 is the same as any one (N01 or Ne') of the effective refractive indexes (N0' and Ne') of the diffraction lattices 20 even when the molecules of the diffraction lattices 20 have not been oriented in a specific direction, one of the λi and λ2 wavelength light can pass through the diffraction lattices 20 without undergoing diffraction while the other can undergo diffraction at the diffraction lattices 20. Here, it is preferable that relatively inexpensive polyimide having excellent productivity, moldability and thermal resistance be used as the polymer. As for the lattice width w and pitch p of the diffraction lattices 20, the diffraction lattices 20 may be formed to have uniform or non-uniform lattice width w equal to or less than about 30 μm to achieve effective refractive indexes (N0' and Ne') and optimized refraction efficiency when the molecules are not oriented. Further, the diffraction lattices 20 may be formed to have an equal pitch or unequal pitches p equal to or less than about 60 μm between the lattices to optimize a diffraction angle. Further, the adhesive material 30 may be at least one of acryl, epoxy, urethane and polyester-based resins, or a polymer thereof. A method of calculating the refractive index (Nefτ) of the adhesive material 30, which can correspond to the effective refractive indexes (N0' and Ne') of the diffraction lattices 20 according to the polarization directions of the λi and λ2 wavelength light (ordinary or extraordinary light) will be discussed based on such properties. By solving a Maxwell equation on the assumption that light is a function of time and a change in the light with time is e*™1, wave equations and boundary conditions for ordinary light (0-wave) mode and extraordinary light (E-wave) mode can be obtained as in the following Table 1. In this time, in order to more simplify these wave equations, χ and γ are introduced as functions of a propagation constant β.

Table 1. Wave equations and boundary conditions for ordinary light (0-wave) mode and extraordinary light (E-wave) mode

Solutions of the wave equations, which are represented by χ and γ defined as in Equation 1, are summarized in Table 2 with respect to areas inside and outside the diffraction lattices 20. At this time, continuous conditions of Ey and Hy were used with respect to the ordinary and extraordinary light modes, respectively, χ and γ indicate a lateral propagation constant and an attenuation constant at a region other than the corrugated portions, respectively.

Table 2. Solutions of wave equations for ordinary light (0-wave) mode and extraordinary light (E- wave) mode

By applying the continuous conditions of H2 and E2 for the extraordinary light (E- wave) mode and the ordinary light (0-wave) mode to the solutions of the wave equations of Table 2 once more, the following relation equations shown in Table 3 can be finally obtained. Propagation constant values for the ordinary light (0-wave) mode and the extraordinary light (E-wave) mode can be obtained from the relation equations of Table 3.

Table 3. Boundary conditions for ordinary light (O-wave) mode and extraordinary light (E-wave) mode

For more simplification and better understanding thereof, a standardization propagation constant b is defined as in the following Equation 2.

The standardization frequency V is a standardization frequency defined in the following Equation 3. The following V-b relation equation can be obtained by arranging and summarizing the relation equations of Table 3 using the standardization propagation constant. At this time, the standardization frequency in typical N-th mode is given as:

, V = (N = 1,2,3, -) (3)

It can be seen from this equation that V and b have a many-to-one correspondence. Here, 7^ is defined as 1 and ^Hr n°' for the ordinary light mode and the extraordinary light mode, respectively. From the above relation equation, a V-b curve may be obtained as Graph 1. Modes in a waveguide may be easily understood from the curve.

Graph 1. Frequency (V) - propagation constant (b) curve for ordinary light (O- wave) mode and extraordinary light (E-wave) mode

Propagation constant (b)

In Graph 1, n means an order of the mode. The number of modes and the values of a and b in each mode can be seen from intersection points between a vertical line corresponding to a given value of V and curves. It can be seen that when V is smaller than π/2, only one mode exists for ordinary light and extraordinary light, respectively, and additional modes are created at a period of π/2 one by one. At this time, b has a value between 0 and 1. The propagation constant and the effective refractive index can be derived from the following Equation 4:

Neff = β/k0 = ^b(ny2 - nc2) + nc2 (4)

The refractive index derived from Equation 4 should be applied to the adhesive material to meet a polarized state. If the refractive index (Neff) of the adhesive material 30 is properly selected using such configurations and derived equations in view of the effective refractive index (N0' and Ne') of the diffraction lattices 20 in order to diffract polarized light of any one of X1 and λ2 wavelengths, light of one of λi and λ2 wavelengths can be diffracted and the other cannot be diffracted. For example, as shown in Fig. 3, the refractive index (Neff) of the adhesive material 30 should be equivalent to the effective refractive index (Ne') of the diffraction lattices 20 to cause light of the X1 wavelength incident on the diffractive optical element 1 to be diffracted and light of the λ2 wavelength not to be diffracted. Accordingly, as shown in Fig. 3a, the X1 wavelength light of the X1 and X2 wavelength light incident on the diffractive optical element 1 is diffracted into 0-th order light and ±lst order light due to a difference between the effective refractive index (N0') of the diffraction lattices 20 and the refractive index (Neff) of the adhesive material 30. Further, as shown in Fig. 3b, the λ2 wavelength light of the X1 and X2 wavelength light incident on the diffractive optical element 1 is not diffracted but is transmitted since the effective refractive index (Ne') of the diffraction lattices 20 is the same as the refractive index (Neff) of the adhesive material 30. Meanwhile, as shown in Fig. 4, the refractive index (Neff) of the adhesive material 30 should be equivalent to the effective refractive index (N0') of the diffraction lattices 20 so that the X2 wavelength light of the X1 and X2 wavelength light incident on the diffractive optical element 1 is diffracted and the λi wavelength light is not diffracted. Accordingly, as shown in Fig. 4a, the light of the λ2 wavelength of the λi and X2 wavelengths incident on the diffractive optical element 1 is then diffracted into 0-th order light and ±lst order light due to a difference between the effective refractive index (Ne') of the diffraction lattices 20 and the refractive index (Neff) of the adhesive material 30. Further, as shown in Fig. 4b, the X1 wavelength light of the X1 and λ2 wavelength light incident on the diffractive optical element 1 is transmitted without undergoing diffraction since the effective refractive index (N0') of the diffraction lattices 20a is equivalent to the refractive index (Neff) of the adhesive material 30. Here, it is assumed that the polarization directions of the X1 and λ2 wavelength light intersect each other before the Xi and λ2 wavelength light is incident on the diffractive optical element 1. This polarization direction may be set in advance by the configuration of the optical pickup device having the diffractive optical element 1 according to this embodiment. Accordingly, the X1 and λ2 wavelength light is selectively incident on the information recording surface of the optical disc, such as DVD or CD, so that information is recorded or reproduced thereon or therefrom. The wave selection type diffractive optical element 1 according to the first embodiment comprises a X1 laser generator for generating laser light of DVD type wavelength (X1), and a X2 laser generator for generating laser light of CD type wavelength (X2), which are separately provided. The diffractive optical element 1 can be disposed on the light path of the CD and DVD compatible optical pickup device that has been already set such that the X1 and X2 polarization directions intersect each other. Alternatively, the wave selection type diffractive optical element 1 may be disposed on the light path of the CD and DVD compatible optical pickup device using a two-wavelength semiconductor laser, wherein X1 and X2 wavelength light with polarization directions intersected at each other is incident on the diffractive optical element 1. Meanwhile, Figs. 5 and 6 are sectional views of a wave selection type diffractive optical element according to a second embodiment of a present invention. As shown in the figures, the wave selection type diffractive optical element 1' according to the second embodiment has a structure in which diffraction lattices 20' are formed on a transparent substrate 10' of a pair of light-transmitting transparent substrates 10' and 11', which is positioned at an exit side of the λ\ and λ2 wavelength light, and an adhesive material 30' is filled between the transparent substrates 10' and 11', in the same manner as those shown in Fig. 1. Further, the diffractive optical element 1' includes a structure in which a phase plate is fixed using the adhesive material 30' between the transparent substrate H1 positioned at a λ\ and λ2 wavelength incidence side and the diffraction lattices 20' so as to rotate a polarization direction by 90° with respect to any one of the X1 and λ2 wavelengths. Here, since the diffraction lattices 20' and the adhesive material 30' are the same as the first embodiment described above, descriptions thereof will be omitted. Meanwhile, the phase plate is made in the form of a film using a thin film of organic material and has a proper optical pass difference (OPD) value. For example, the phase plate has an OPD value selected such that a polarization direction of light with a wavelength (λ{) of 650nm is rotated by 90° and light with a wavelength (λ2) of 790 nm is directly transmitted without an optical pass difference. Here, the optical pass difference for the rotation of the polarization directions of the light with λ\ and λ2 wavelengths may be reverse. At this time, the most proper OPD value may be determined from the following Equation 5. The simplest method is to fabricate a phase plate with an OPD value corresponding to a half waveplate (a retardation plate rotating a polarization direction by 90° with respect to a polarization plane) for X1 and with an OPD value corresponding to a full waveplate (a retardation plate rotating the polarization direction by 180° with respect to the polarization plane) for λ2.

dl = (2χml+l) x λϊ/2 d2 = m2 χ λ2 (5)

where ml and m2 are integers (i.e., 1, 2, 3...), and d2 has the properties of a full waveplate if dl has an OPD value of a half waveplate for λ\. It is desirable to select ml and m2 such that a difference between dl and d2 is substantially close to zero. As a result of calculation, it is found that the most preferable OPD value is obtained when each of ml and m2 is 2 and an allowed range of the OPD value is between 1580 and 1620 nm. In particular, it is found that the most preferable property is obtained when the OPD value is about 1600 nm± 20 nm. With such a structure of the wave selection type diffractive optical element I1 according to the second embodiment, when the λi and λ2 wavelength light incident in the same polarization direction pass through the phase plate, the polarization direction of the λj wavelength light is rotated by 90° and the λ2 wavelength light is directly transmitted without phase retardation, resulting in the intersection of the X1 and λ2 wavelength light. To diffract the polarized light of any one of the intersecting λi and λ2 wavelength light, the refractive index (Neff) of the adhesive material 30' is properly selected in view of the effective refractive index (N0' and Ne') of the diffraction lattices 20' in the same manner as the first embodiment described above. Thus, light of any one of the X1 and λ2 wavelengths can be diffracted while light of the other cannot be diffracted. Since the diffraction operation for the X1 and λ2 wavelength light according to the second embodiment is the same as the first embodiment as shown in Figs. 5a and 5b and Figs. 6a and 6b, a description thereof will be omitted. According to the second embodiment, as shown in Fig. 2, the wave selection type diffractive optical element 1' may be disposed on a light path of a CD and DVD compatible optical pickup device, which uses a two-wavelength semiconductor laser that generates X1 and λ2 wavelength light with an identical polarization direction. Meanwhile, Fig. 7 is a sectional view of a wave selection type diffractive optical element according to a third embodiment of the present invention. As shown in Fig. 7, the wave selection type diffractive optical element 1 " according to the third embodiment has a structure in which a first transparent substrate 10", a second transparent substrate 12", and a third transparent substrate 13" are sequentially disposed from an exit side of X1 and λ2 wavelength light (i.e., a side directed to an optical disc; a lower portion in the figure) to an incidence side of the X1 and λ2 wavelengths (an upper portion in the figure), and first diffraction lattices 20" and second diffraction lattices 22" are formed on surfaces (upper surfaces in the figure) of the first and second transparent substrates 10" and 12" facing the X1 and λ2 incidence side, respectively. A phase plate 40 is disposed between the second and third transparent substrates. A first adhesive material 30" is filled between the first and second transparent substrates 10" and 12" to fix the first diffraction lattices 20". Further, a second adhesive material 32" is filled between the second transparent substrate and the phase plate to fix the second diffraction lattices 22", and a third adhesive material 33" is filled between the phase plate and the third transparent substrate to fix them to each other. Here, the structures of the first and second diffraction lattices 20" and 22" and the phase plate are the same as the first and second embodiments. The refractive index and the material of the first to third adhesive materials 30" to 33" are also the same as the first and second embodiments. Accordingly, descriptions thereof in the third embodiment will be omitted. In the wave selection type diffractive optical element 1" with such a structure according to the third embodiment, light of one of the λj and λ2 wavelengths may be directly transmitted or diffracted at any of the diffraction lattices by properly selecting the refractive index (Neff) of the first to third adhesive materials 30" to 33" in view of the effective refractive index (N0' and Ne') of the first and second diffraction lattices 20" and 22". For example, the refractive index (Neff_) of the second adhesive material 32" should be different from the effective refractive index N0' of the second diffraction lattices 22" with respect to the X1 wavelength (No'≠Neffi), and the refractive index (Nefπ) of the first adhesive material 30" should be equivalent to the effective refractive index (N01) of the first diffraction lattices 20" with respect to the so that the X1 wavelength light of the X1 and λ2 wavelength light incident on the diffractive optical element 1 " is diffracted at the second diffraction lattices 22" while the X2 wavelength light is transmitted at the second diffraction lattices 22" and is then diffracted at the first diffraction lattices 20", as shown in Figs. 7a and 7b. Accordingly, the relationship between the effective refractive index (Ne') of the second diffraction lattices 22" and the refractive index (Hem) of the second adhesive material 32" with respect to the λ2 wavelength becomes Ne=NeE, and the relationship of the effective refractive index (Ne') of the first diffraction lattices 20" and the refractive index (Neff0 of the first adhesive material 30" with respect to the λ2 wavelength becomes Ne'≠Neffl. As a result, as shown in Fig. 7a, the polarization direction of the X1 wavelength light of the incident λ\ and λ2 wavelength light is rotated at the phase plate by 90° and is diffracted into 0-th order light and ±lst order light due to the difference between the effective refractive index (N0') of the second diffraction lattices 22" and the refractive index (Neff_) of the second adhesive material 32" (N0'≠Neffl) when passing through the second diffraction lattices 22". The λ\ light is not further diffracted at the first diffraction lattices 20" since the effective refractive index (N0') of the first diffraction lattices 20" is equivalent to the refractive index (Nefn) of the first adhesive material 30" (N0 -Nefπ) and is transmitted while maintaining three beams of the 0-th order light and the ±lst order light. The three beams are irradiated onto a relevant optical disc (e.g., DVD) so that the information is recorded in or reproduced from the optical disc. Meanwhile, as shown in Fig. 7b, the diffraction direction of the λ2 wavelength light of the incident λl and λ2 wavelength light is not rotated at the phase plate, and is transmitted without undergoing diffraction when passing through the second diffraction lattices 22" since there is no difference between the effective refractive index (Ne') of the second diffraction lattices 22" and the refractive index (Nsm) of the second adhesive material 32" (Ne'^Nefs) with respect to the λ2 wavelength. The λ2 wavelength light is then diffracted into three beams of 0-th order light and ±lst order light at the first diffraction lattices 20" due to the difference between the effective refractive index (Ne') of the first diffraction lattices 20" and the refractive index (N6Si) of the first adhesive material 30" (Ne'≠Neffi) with respect to the λ2 wavelength. The three beams are incident on a relevant optical disc (e.g., CD) so that the information is recorded in or reproduced from the optical disc. The wave selection type diffractive optical element 1" according to the third embodiment is disposed on the light path of the CD and DVD compatible optical pickup device that uses a two-wavelength semiconductor laser for generating X1 and λ2 wavelength light with the same polarization direction, as shown in Fig. 2, so that the polarization directions of the λi and λ2 wavelength light intersect each other and the X1 and λ2 wavelength light can be irradiated selectively onto the relevant optical disc (e.g., DVD or CD) as the three beams. As described above, in the wave selection type diffractive optical element and the optical pickup device having the optical element according to the present invention, the diffraction lattices are made of a non-liquid crystal-based polymer material in a corrugated shape in a state where molecules are not oriented on the surface of the transparent substrate. Further, the refractive index of the adhesive material is selected to be identical to or different from the effective refractive index of the diffraction lattices with respect to any one of the λj and λ2 wavelengths. Thus, any one of the X1 and λ2 wavelength light can be simply and selectively diffracted without orienting the optical molecules of the diffraction lattices. Accordingly, the diffractive optical element and the optical pickup device having the diffractive optical element are simply manufactured and production costs thereof are significantly reduced. Further, the refractive index of the adhesive material can be exactly calculated with respect to the effective refractive index of the diffraction lattices, so that a deviation in the refractive index can be minimized to improve diffraction efficiency and a deviation in the quality of the products can be minimized. Accordingly, the reliability of the products is improved.

INDUSTRIAL APPLICABILITY As described above, the present invention provides a wave selection type diffractive optical element and an optical pickup device having the same, wherein the diffractive optical element can be easily manufactured, production costs thereof can be reduced, and a deviation in a refractive index can be minimized, thereby improving the quality of the products.